Friction-type rod joint



April 28, 1970 C, 1 COBERLY ET AL 3,508,773

FRICTION-TYPE ROD JOINT Original Filed NOV. 27, 1965 .7 NVENTORS.

BY Tye/,9 rraeueyb United States Patent O 3,508,773 FRICTION-TYPE ROD .IOINT Clarence J. Coberly, San Marino, and Francis Barton Brown, La Crescenta, Calif., assignors to Kobe, Inc., Huntington Park, Calif., a corporation of California Continuation of application Ser. No. 326,408, Nov. 27, 1963. This application June 12, 1967, Ser. No. 645,549 The porti-on of the term of the patent subsequent to Dec. 17, 1980, has been disclaimed Int. Cl. F16d 1/02 U.S. Cl. 287-108 13 Claims ABSTRACT OF THE DISCLOSURE A high-strength, friction-type joint for interconnecting rods, such as sticker rods for oil well pumping, in end-toend relation to form a rod string capable of withstanding high axial tension loads. Adjacent, tapered rod ends converging axially toward each other are telescoped into complementarily tapered ends of a coupling frictionally connecting the rods. The axial lengths of the tapered interfaces between the rod ends and the coupling ends, the engagement pressures between the coupling ends and the rod ends in such interfaces, and the effective coeiiicients of friction between the coupling ends and the rod ends in such interfaces, are so related as to provide frictional resistances to relative bodily displacements of the coupling and rod ends at least nearly equal to the yield strength of the body portions of the rods.

This application is a continuation of application Ser. No. 326,408 tiled Nov. 27, 1963 and now abandoned.

BACKGROUND OF THE INVENTION Under static conditions, or during slow-speed operation, the axial tensile stress in a sucker rod string used in oil well pumping has two components, the first being due to the weight of the string and the second being due to the load on the pump. Assuming no rod-diameter variation With well depth, the axial tensile stress due to the weight of the rod string increases directly with depth and reaches values approaching the yield points of currentlyused rod materials at depths of from 10,000 to 15,000 feet, the maximum stress occurring at the upper end of the string. Therefore, the pump load that can be sustained by the rod string decreases with depth and becomes zero When the axial tensile stress at the upper end of the rod string which is due to the weight of the string alone becomes equal to the allowable working stress. To increase the operating depth, it is common to progressively decrease the diameter of the rod string with depth, thereby reducing the weight which must be carried by the largest-diameter rods at the upper end of the string.

Under dynamic conditions, the acceleration of the rod string increases the axial tensile stress at the bottom of the pumping stroke and decreases it at the top. However, such stress variations do not travel throughout the length of the rod string instantaneously with the result that the stress may be out of phase with the rod string motion`at the well head. Thus, the rod string has a natural frequency which may be within the range of pump speeds used in deep wells. If the rod string is operated at or near its natural frequency, or at a harmonic thereof, the axial tensile stress may be greatly intensified.

Sucker rod joints must be capable of withstanding such high, transitory stresses, in addition to the stresses due to the Weight of the string and the pump load thereon. Furher, conventional threaded joints must be capable of withstanding the additional concentrations of stresses occurring at the thread roots, or at other points of discontinuity. To produce a joint which will withstand such stresses ice and which will have a fatigue strength equal to that of the rods themselves, i.e., that of the bodies of the rods, the rods must be considerably upset and the minimum cross sectional area at the thread roots -must be greater than the rod area. In present threaded joints, the minimum cross sectional area is 50% to 75% greater than the rod area itself. ISimilar considerations are applicable to the coupling of the conventional threaded joint, which must have a high wall thickness to carry the various sustained and transitory loads imposed thereon and to over come the effects of stress concentrations at the thread roots. The result of the large rod-end upsets and the large coupling wall thickness required by conventional threaded sucker rod joints is a joint having an outside diameter more than double the nominal rod diameter, i.e., more than double the diameter of the body of the rod.

Since the rod string is operated inside the tubing through which the oil is pumped to the surface, the joints offer restrictions which often limit the productive capacity of the pumping system. In order to achieve an adequate capacity, the production tubing in which the rod string operates frequently must be larger than necessary to receive the pump itself. This, in turn, may require the drilling of an oversize well bore simply to accommodate the large-diameter threaded joints.

Another problem encountered with conventional threaded joints in current usage is that they sometimes unscrew in operation. Various expedients have been developed to overcome this problem, such as providing means for constantly rotating the rod string at the well head ina direction to continuously tend to tighten the joints. Such expedients increase the complexity of the pumping installation and are often not fully effective.

SUMMARY AND OBJECTS OF THE INVENTION The present invention overcomes the foregoing and various other drawbacks of conventional threaded joints by providing a joint which relies solely on friction between telescopically related, mating tapered surfaces to achieve working and fatigue strengths at least as great as the working and fatigue strengths of the associated rod itself, the provision of such an unthreaded joint being a basic object of the invention.

Other important objects are to provide such a highstrength, friction-type rod joint which has an outside diameter much less than that of a corresponding threaded joint, which is less expensive than a corresponding threaded joint, and which will not loosen and drop the rod string therebeneath under the most severe service conditions.

More particularly, the present invention contemplates a high-strength, friction-type rod joint comprising telescopable inner and outer members, one of which is an end portion of a rod and the other of which is an end portion of a coupling, respectively having complementarily tapered outer and inner surfaces frictionally interengaged to such an extent as to develop a joint strength at least substantialy equal to the yield strength of the rod itself, and ranging upwardly to a value exceeding the ultimate strength of the rod. Preferably, the end portion of the rod constitutes the inner member of the joint, the coupling being tubular and constituting the outer member thereof.

Other objects of the invention are to provide a highstrength, friction-type rod joint of the foregoing nature wherein:

The rod end forming part of the joint requires a smaller upset than for a threaded joint, and the coupling requires a smaller wall thickness than for a threaded joint, therelby reducing the outside diameter of the joint materially as compared to a threaded joint.

Discontinuities of cross sectional area and discontinuities of stress axially of the joint are minimized.

The desired high frictional resistance to relative bodily displacement of the tapered surfaces of the joint is achieved by a high engagement pressure between such surfaces.

The desired high engagement pressure between the tapered surfaces of the joint is induced by a shrink 'fit lbetween the inner and outer members of the joint, e.g., between the rod end and the coupling end telescoped thereover.

The high engagement pressure between the tapered surfaces induced by the shrink iit therebetween is the result of a high hoop tension stress in the outer member close to but below the yield point of the material of the outer member, and an opposing high compression stress in the inner member close to but below the yield point of the material of the inner member.

The axial length of the interface between the tapered surfaces, i.e., the axial length of the tapered surfaces in pressural interengagement with each other, is so related to the engagement pressure between the tapered surfaces and the effective coefficient of friction therebetween as to produce a frictional resistance to relative bodily displacement ofthe tapered surfaces suiciently high to develop a joint strength at least nearly equal to the yield strength of the body of the rod, and ranging upwardly to a value in excess of the ultimate strength of the body of the rod.

The desired high engagement pressure between the tapered surfaces is produced hydraulically.

The desired high engagement pressure between the tapered surfaces is produced 'by hydraulically expanding the outer member and hydraulically contracting the inner member, then moving the members axially together while the outer is hydraulically expanded and the inner hydraulically contracted, and finally permitting the outer and inner members to contract and expand, respectively.

The joint may be assembled by initially inserting the inner member into the outer, applying hydraulic pressure to an axially central region of the interface between the tapered surfaces to expand the outer member and contract the inner member in such central region while axially moving the members together into a final inserted position of the inner member to continuously maintain the tapered surfaces in sealing engagement in axially spaced sealing regions at opposite ends of the central region, and then relaxing the hydraulic pressure within the central region of the interface to permit the outer and inner members to contract and expand, respectively, in the central region so as to bring the tapered surfaces thereof into pressural interengagement with the desired high engagement pressure therebetween.

The joint may be broken hydraulically following a procedure which is essentially the reverse of the preceding. One or both members of the joint are so formedA adjacent the larger ends of the tapered surfaces thereof as to avoid any abrupt stress change in the rod end due to the shrink fit of the outer member onto the inner.

The ratio of the length of the interface between the tapered surfaces to the diameter of the body of the rod, the effective coeiiicient of friction between the tapered surfaces, and the included taper angle of the tapered surfaces, all fall within ranges which will result in a joint strength up to the ultimate strength of the body of the rod, when the hoop tension and compression stresses in the outer and inner members are close to the yield points of the respective materials thereof.

More particularly, the ratio of the axial length of the interface to the diameter of the body of the rod falls between about `0.5 and about 6.0, the effective coefficient of friction falls between about 0.1 and about 0.8, and the included taper angle of the tapered surfaces falls between about 30 and about 18.

The joint is so constructed that the outer member will grip the inner member more tightly upon application of an axial tension load to the joint.

The materials and dimensions of the inner and outer members are so selected and related that at least a portion of the outer member contracts relative to the inner member upon application of an axial tension load to thereby increase the engagement pressure between the tapered surfaces adjacent such portion.

Various other important factors enter into the construction of a high-strength, friction-type rod joint embodying the invention. One is that the engagement pressure between the tapered surfaces of the inner and outer members must not exceed one-half the yield point of the material of the inner member in compression. The reason for this is that, with a solid inner member, the compressive stress induced on the axis of the inner member by the shrink t of the outer member on the inner is twice the engagement pressure between the members. Consequently, the joint of the invention must achieve strengths up to the ultimate of the rod lwithin this limit on the engagement pressure.

Considering another factor, the high hoop tension stress in the outer member of the joint establishes therein an initial axial compression stress which, according to Poissons ratio for steel, is approximately equal to 0.3 times the hoop tension stress. Such an initial axial compression tends to reduce the axial length of the outer member and tends to increase its taper angle to a related extent. Conversely, the opposing high compression stress in the inner member establishes therein an initial axial tension equal to about 0.3 times the stress in question. This tends to lengthen the inner member and tends to decrease its taper angle. However, it has been found that the actual taper-angle changes are insignificant and do not affect the joint strength signicantly.

Still another factor is that in applying an axial make up force to the members in making up the joint, the inner member may be subjected to an axial compressive stress tending to increase its diameter. Upon relaxation of the make up force, the diameter of the inner member tends to decrease to its original value, which would appear to tend to weaken the joint by reducing the engagement pressure. However, this effect has been found to be insignicant.

An external axial load applied to one of the members of the rod joint is transferred progressively to the other along the length of the interface throughout which the two tapered surfaces are pressurally interengaged. Stated in other words, the axial stress due to an external axial load varies along the interface from to 0% in one of the members and from 0% to 100% in the other. An axial tensile stress due to an axial tension load is additive with respect to the initial axial tension stress in the inner member. Likewise, the axial tension stress due to such an axial load reduces the initial axial compression stress in the outer member to the point of changing it to a net axial tension stress along most of the length of the interface.

The engagement pressure between the tapered surfaces induced by the hoop tension and compression stresses in the outer and inner members changes upon application of such an axial load to one of the members. If it is assumed that initially the engagement pressure or interference lit between the surfaces was uniform along the interface, application of such an axial load will increase the contact pressure or interference t near one end of the interface and decrease same near the other end thereof. At some intermediate point these effects cancel and the contact pressure is the original applied pressure. Likewise, application of such an axial load has been found to cause relative axial movement between the engaged tapered surfaces at positions near the ends of the interface.

It thus becomes apparent that any analysis of the stresses in the joint under initial and loaded conditions becomes extremely complex. Many of the factors noted above as concerns initial stress and change in stress upon axial loading might seem to indicate that no friction-type joint could be designed that would not pull apart at loads even as low as the nominal yield strength of the associated rod, to say nothing of loads exceeding the nominal ultimate strength of the rod. Tests have shown, however, that the joint of the invention can be designed to meet these conditions without failure. It has been found that the change in axial stress in the members due to axial loading, change in engagement pressure or interference fit between the tapered surfaces upon such loading, and the relative movement between such surfaces upon such loading, are not such as to prevent the design of a friction-type joint having a strength greater than the yield or even the ultimate strength of the rod.

The foregoing objects, advantages, features and results of the present invention, together with various other objects, advantages, features, and results thereof which will be evident to those skilled in the art in the light of this disclosure, may be achieved with the exemplary embodiments of the invention described in detail hereinafter and illustrated in the accompanying drawing.

DESCRIPTION OF THE DRAWING In the drawing:

FIG. l is a longitudinal sectional view of a highstrength, friction-type rod joint construction, utilizing an external coupling, which embodies the invention;

FIGS. 2, 3 and 4 are fragmentary longitudinal sectional views illustrating, on greatly enlarged scales, various constructions for providing high effective coefiicients of friction in the rod joint construction of FIG. l;

FIG. 5 is a fragmentary longitudinal sectional view similar to a portion of FIG. 1, but illustrating an alternative way of preventing stress concentration;

FIG. 6 is a fragmentary longitudinal sectional view similar to a portion of FIG. l, but illustrating thread means for making up the friction-type rod joint of the invention; and

FIG. 7 is an enlargement of a portion of the thread means of FIG. 6.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION Referring initially to FIG. 1, illustrated therein is a friction-type rod joint construction of the invention comprising an external coupling 22 the respective end portions or ends 24 and 26 of which receive therein upset end portions or ends 28 and 30 of sucker rods 32 and 34 forming a sucker rod string 36, the upset rod ends being relatively closely spaced within the coupling to minimize the over-all length of the joint construction. The coupling and rod ends 24 and 28 form a first rod joint 38 and the coupling and rod ends 26 and 30 form a second rod joint 40. The two joints 38 and 40 are identical so that only the former will be considered.

The inner member of the rod joint 38, i.e., the upset rod end 28, is provided with a tapered outer surface 42 which converges axially inwardly relative to the coupling 22. The outer member of the joint 38, i.e., the coupling end 24, is provided with a complementarily tapered inner surface 44. The two tapered surfaces 42 and 44 are pressurally interenaged along the interface therebetween with a high engagement pressure induced by a high hoop tension stress in the coupling end 24 and an opposing high compression stress in the rod end 28. As will be discussed in detail hereinafter, the axial length of the tapered surfaces 42 and 44 in pressural interengagement with each other is so related to the high engagement pressure between the tapered surfaces and the effective coefficient of friction therebetween as to produce a frictional resistance to relative bodily displacement of the tapered surfaces, either axially or crcumferentially, sufiiciently high to develop a joint strength ranging from one at least nearly equal to the yield strength of the body of the rod 32 upwardly to one exceeding the ultimate strength of the rod body.

The foregoing high frictional resistance to relative bodily displacement of the tapered surfaces 42 and 44 is preferably achieved by shrinking the coupling end 24 onto the upset rod 28, and particularly by hydraulically shrinking the coupling end onto the rod end. Considering how this may be accomplished, the coupling end 24 is provided at approximately the axial midpoint of its tapered surface 44 with a port l46 for injecting a fluid, such as oil, under high pressure into the axially central region of the annular interface between the tapered surfaces 42 and 44. The port 46 communicates at its inner end with an internal annular groove 48 in the coupling end 24, and is provided at its outer end with a radiallyinwardly-convergent annular seat 50 for a suitable fluid injection nozzle, as shown in our Patent No. 3,114,566, granted Dec. 17, 1963.

In making up the rod joint 38, the upset rod end 28 is inserted in the coupling end 24 until the tapered surface 42 engages the tapered surface 44 in the central region of the interface therebetween and in annular sealing regions on axially opposite sides of the port 46. Oil, or other fluid, under high pressure, which pressure may be as high as 30,000 p.s.i., or more, is then injected into the annular central region of the interface between the two tapered surfaces, at the same time applying an axial make up force to the coupling 22 and the rod 32 tending to further insert the rod end 28 into the coupling end 24, as by means of apparatus disclosed in our aforementioned patent. The high uid pressure within the central region of the interface expands the adjacent portion of the coupling end 24 outwardly and contracts the adjacent portion of the rod end 28, without, however, breaking contact in the annular sealing regions on axially opposite sides of the central region so long as a sufficiently high axial make up force is applied. This make up force must be high enough to resist the action of the injection pressure on the axially projected areas of the tapered surfaces 42 and 44. Thus, the injected 'Huid is trapped in the annular central region of the interface.

As the pressure of the trapped fiuid builds up, the coupling and rod ends 24 and 28 are caused to be moved axially toward each other by the axial make up force to increase the extent to which the rod end is inserted into the coupling end. (The coupling 22 is provided therein with a bleed port 52 to drain from the space between the rod ends 28 and 30 any injected fluid leaking into such space so as to prevent a pressure build-up therein from interfering with making up of the joint 38,

or with its strength.) After the maximum injection pres-- sure and the maximum axial make up force for which the rod joint 38 is designed have been reached, the rod end 28 is in effect bottomed within the coupling end 24. v(Such bottoming is solely the result of interengagement of the tapered surfaces 42 and 44, there being no annular shoulders, or the like, to artificially limit insertion.) Then, the application of the injection pressure is discontinued and the excess injected fluid is permitted t0 escape through the port 46. This permits the outwardly expanded portion of the coupling end 24 to contract inwardly, and simultaneously permits expansion of the contracted portion of the rod end 28. The result is that the coupling end 24 is shrunk onto the rod end 28 with a high engagement pressure determined by the maximum injection pressure and the maximum axial make up force.

It will be apparent that, in order to break the joint 38, a similar procedure is followed. The injection pressure acting on the axially projected areas of the tapered surfaces 42 and 44 is normally sufficient to produce axial separation of the coupling and rod ends 24 and 28. Actually, it may `be necessary to restrain such axial separation.

When making up and breaking the joint 38, the major portions of the tapered surfaces 42 and 44 intermediate the ends thereof are physically separated by the injected fluid, the latter being trapped in the central region of the interface with contact pressures between the tapered surfaces at the ends of the interface which are not excessively high. Consequently, galling of the tapered surfaces 42 and 44 in response to relative axial movement thereof in making up and breaking the joint 38 does not occur despite very high engage-ment pressures 'between the tapered surfaces when the joint is made up. Thus, the joint 38 may be made up and broken repeatedly.

An alternative way of making up the rod joint 38, which normally would be used only when the joint is to be permanent, is to jab the rod end 28 into the coupling end 24, with a suitable lubricant between the tapered surfaces 42 and 44, by means of a sudden application of energy suiciently high to produce the desired hoop tension and compression stresses in the coupling and rod ends, thereby producing the desired high engagement pressure between the tapered surfaces. To prevent galling of the tapered surfaces 42 and 44 in using this jab method of making up the joint 38, the nature of the lubricant applied to the tapered surfaces must be such as to prevent extrusion of all of it out of the interface. Suitable lubricants are highly viscous liquids or heavy greases. Such lubricants may have mixed therewith suitable keying particles adapted to embed themselves in the tapered surfaces 42 and 44 to increase the effective coefficient 0f friction therebetween, as will `be discussed hereinafter in connection with FIG. 4. Various energy sources capable of producing a sudden energy output suiiiciently high to induce the desired hoop tension and compression stresses in the coupling and rod ends 24 and 28 may be used. Examples are an explosive charge, a high pressure gas-type accumulator, a high pressure liquid-type accumulator, and the like.

With the foregoing as background, various important considerations of the invention which enter into the structure of the rod joint 38, and into the materials used for the coupling 22 and the rod 32, will now be discussed.

In general, the axial length of the tapered surfaces 42 and 44 in pressural interengagement with each other, the engagement pressure between the tapered surfaces resulting from the hoop tension and compression stresses in the coupling and rod ends 24 and 28, and the effective coefficient of friction between the surfaces, are so related as to produce a frictional resistance to relative bodily displacement of the tapered surfaces sufficiently high to develop a high-strength friction-type rod joint 38, the joint strength being at least nearly equal to the nominal yield strength of the rod 32, i.e., the yield strength of the body of the rod, and, under some conditions, even exceeding the ultimate strength of the rod.

More particularly, the joint strength may be increased by increasing the axial length of pressural interengagement between the tapered surfaces 42 and 44, the effective coeicient of static friction therebetween, or the engagement pressure therebetween. To avoid an excessively long joint 38, the axial length of the tapered surfaces 42 and 44 is preferably kept as small as possible, which means that it is necessary to make the engagement pressure and the effective coefficient of friction as high as possible (at the same time keeping the included angle of the tapered surfaces relatively small).

Considering the matter of the axial length of the interface between the tapered surfaces 42 and 44 in more detail, this length must be at least about 0.5 times the diameter of the body of the rod 32 to obtain the desired high joint strength. However, the maximum axial length of the interface may be as much as about 6.0 times the diameter of the rod body without being excessive for some applications, but is preferably not more than about 3.0 times the diameter of the rod body to be commercially practicable for all applications.

Considering another approach to the matter of the axial length of the interface, it is necessary in most instances to regard the coupling end 24 as a thick walled member the hoop tension stress in which varies radially. The maximum value of this hoop tension stress must be considered. The rod end 28 is not critical, except that the hydraulic pressure to expand the coupling end, and the engagement pressure between the coupling end, and the rod end, cannot be more than one-half the compressive yield stress of the material used for the rod 32. Considering the rod 32 as having the upset end 28, then the length of pressural interengagement required for any set of conditions may be determined in accordance with the equation where L is the axial length of the interface between the tapered surfaces 42 and 44,

SR is the axial tensile stress in the body of the rod 32 resulting from an axial load thereon,

SC is the maximum hoop tension stress in the coupling end 24 at the axial midpoint of the interface,

dR is the nominal diameter of the rod, i.e., the diameter of the body of the rod,

d1 is the outer diameter of the coupling at the axial midpoint of the interface, 1

d2 is the inner diameter of the coupling at the axial midpoint of the interface,

f is the eflective coefficient of friction between the tapered surfaces, and

a is the included taper angle of the tapered surfaces.

In some instances, e.g., for a small-diameter rod joint 38 utilizing a relatively thin coupling 22, the foregoing thick-walled equation may be simplied and replaced, without serious error, by the thin-walled equation As the foregoing equations suggest, the axial length of the tapered surfaces 42 and 44 in pressural interengagement in the made up rod joint 38 may be minimized in various ways. One way is to utilize hoop tension and compression stresses in the coupling and rod ends 24 and 28 which are as high as possible, thereby achieving as high an engagement pressure between the tapered surfaces as possible. Preferably, in the absence of applied loads, these stresses are close to but slightly less than the respective yield points of the materials of which the coupling and rod ends 24 and 28 are made, in which event SR and Ss may be taken as the respective yield points of the rod and coupling materials. With stresses close to the yield points, the value of L in the foregoing equations becomes the minimum possible interface length, all else being equal. However, even with such high hoop tension and compression stresses in the coupling and rod ends, the interface length may run as high as three times the minimum, i.e., three times the smallest possible value of L. Also, with stresses close to the yield points, the interface length to rod-body diameter ratio discussed above may be held within the preferred range of 0.5 to 3.0.

Considering some factors involved in the selection of materials for the coupling 22 and the rod 32, the ratio SR/SC in the foregoing equations is preferably not more than 1.00, based on the yield points of the respective materials in solving for the minimum value of L. By using a high strength material for the coupling 22, e.g., a material having a strength about 50% higher than that of the rod 32, the ratio SR/SC may be of the order of 0.67. A further reduction to as low as 0.50 is possible. To minimize the outside diameter of the rod joint 38, it is desirable to use as high strength a material as possible for the coupling 22 since, in view of the fact that it is a relatively small and light member, its cost is not a significant factor in the total cost of the rod string 36. Also, by using a hard, nongalling material for the coupling 22, wear as the result of rubbing against a tubing string in which it is disposed is minimized. Preferably, a nitrided steel is used for the coupling 22, such a material having a high strength combined with a very high surface hardness and resistance to galling in combination with either nitrided steel or other metals.

Considering another approach to determining the structural characteristics of the rod joint 38, it will be recalled, as stated above, that the contact pressure between the tapered surfaces 42 and 44 cannot exceed SR/Z, Where SR is the yield point of the material of the rod end 28. Consequently, the yield point of the coupling material must be at least equal to the value of SC in the equation.

C 2 a2-df (iii) d1 and d2 having been defined previously. If d1 is taken as cldR, dR having been defined above, and d2 is taken as cgdR, Equation III becomes Because of the effect of Po-issons ratio, c2 must be at least 1.14, and a value of 1.2 is preferred as a reasonable, practical minimum. Assuming a value of 1.8 for c1, for purposes of illustration, Equation IV shows that or that SR/SC=0.77, this value of SR/SC being within the range discussed above. Similarly, d1 can be determined, for any value of SR/SC, from the equation Substituting the foregoing preferred minimum of 1.2 for c2 in Equation V, it can be shown for example, that cl1=1.70dR when SR/SC=2/3, and that d1=l.55dR when SR/Sczl/z. This demonstrates how the over-all diameter of the joint 38 may be reduced by reducing the ratio SR/SC, i.e., by utilizing a high strength material for the coupling 22.

The other principal factor determining the required minimum length of the interface between the tapered surfaces 42 and 44 is, as hereinbefore indicated, the effective Coefficient of friction, f, which is preferably made as large as possible. With the materials normally used for rods and couplings in the oil industry, and with the surface roughnesses normally encountered, the effective coefficient of friction is in the neighborhood of 0.20. However, in some instances, a value as low as about 0.1 may be used without departing from the interface length to rod-body diameter ratio ranges given above. Also, effective coefficients of friction much higher than this, up to as high as the order of 0.80, can be achieved. For example, either or both of the tapered surfaces 42 and 44 may be roughened artificially, as by knurling, etching, sand blasting, plating in such a Way as to roughen, and the like. Referring to FIG. 2 of the drawing, one of the tapered surfaces 42 and 44, preferably the tapered surface 44 of the coupling end 24, is shown as roughened by boring this coupling end with a tool shape and feed which will provide the tapered surface 44 with the equi-valent of a fine thread 54 of buttress form and a sharp crest. Such thread 54 will greatly increase the unit contact pressure and may even slightly deform, temporarily, the mating tapered surface 42, while the two tapered surfaces 42 and 44 are pressurally interengaged throughout the substantially continuous interface therebetween (as suggested in FIG. 2), to provide the effect of an extremely shallow threaded engagement between the two tapered surfaces. (Preferably, as shown in FIG. 2, the thread 54 does not completely embed in the tapered surface 42 to avoid permanently deforming it.) This, however, does not interfere with making up and breaking the rod joint 38 without relative rotation of the coupling and rod ends 24 and 28. Even using a depth for the thread 54 of as little as 0.001 inch, or less, the effective coefficient of friction between the two tapered surfaces is increased to in the neighborhood of 0.40. (In this connection, it should be pointed out that the thread 54 is very greatly magnified in FIG. 2.) Consequently, all else being equal, this cuts in half the length of the interface between the tapered surfaces 42 and 44 which is required to achieve a given joint strength.

As shown in FIG. 3, the effective coefficient of friction may Vbe increased by inserting between the tapered surfaces 42 and 44 a thin film 56 carrying a keying agent 58 which tends to embed itself into the tapered surfaces under the effect of the pressural interengagement therebetween. (In this case, pressural interengagement between the two tapered surfaces 42 and 44 occurs without direct physical Contact, the tapered surfaces remaining physically separated by the film 56.) The film 56 may be of the order of 0.001 inch in thickness, and the keying agent 58 may comprise sharp-edged wire of diamond cross section carried by the film 56, the maximum transverse dimensions of the wire being of the order of 0.010 inch. The keying agent 58 may comprise a single, helical piece of wire, or it may comprise a plurality of pieces of wire. With this construction, the effective coefficient of friction may also be increased to in the neighborhood of 0.40.

Turning to FIG. 4, illustrated therein is a keying agent comprising particles 60 of a hard material, such as tungsten Carbide, or the like, which are disposed between and which tend to embed themselves in the tapered surfaces 42 arid 44 (which may be in actual physical Contact between the keying particles). The keying particles 60 preferably have maximum transverse dimensions of the order of 0.002 inch and will increase the effective coefficient of friction between the tapered surfaces to of the order of 0.50. The keying particles 60 may be introduced into the interface between the tapered surfaces 42 and 44 in the fluid used in shrinking the coupling end 24 onto the rod end 28, or in the lubricant used in jabbing the rod end into the coupling end. Alternatively, they may be carried by a film corresponding to the film 56, in which case the film prevents direct physical contact between the tapered surfaces. Another alternative, for a permanent rod joint, is to use as a fluid for shrinking the coupling end 24 onto the rod end 28 an adhesive which may also have keying particles similar to the keying particles 60 suspended therein. For example, an epoxy resin might be used. With this construction, an extremely high effective coefficient of friction can be achieved.

The included angle a of the tapered surfaces 42 and 44 enters into the axial length of engagement of the surfaces, but only to the extent of reducing the influence of the effective coeicient of friction by the tangent of onehalf its value. Theoretically, the included taper angle could be 0, but, as a practical matter, to facilitate insertion of the rod end 28 into the coupling end 24, and t0 limit the variation in depth of insertion of the tubing end into the coupling with practical diameters and angle tolerances the included angle taper should be not less than about 030. The maximum included taper angle should not be more than about 8 where, as shown in FIG. 1, the wall thickness of the coupling end 24 varies in the axial direction as the result of forming the tapered surface 44 by machining. It will be noted that the wall thickness of the coupling end 24 decreases in the direction of divergence of the tapered surface 44. This reduction in wall thickness should be limited to about 20%.

However, the Wall thickness of the coupling end 24 may be kept constant (except for the external taper 55 to be described) by aring the entire coupling end 2.4 with the desired taper. Since the coupling end 24 thus has a constant wall thickness, it is capable of withstanding a higher hoop tension. The effect of this is that the included taper angle of the surfaces 42 and 44 may be higher than with the tapering wall thickness of the coupling end 24 shown. With such a construction, the included taper angle may range from 30 to as high as in the neighborhood of 18, the upper limitation being determined primarily by the maximum permissible outside diameter of the joint.

Turning now to a discussion of still other important considerations involved in the rod joint 38, it will be recalled that the high hoop tension and compression stresses in the coupling and rod ends 24 and 28 respectively establish axial compression and tension stresses in the coupling and rod ends which, according to Poissons ratio, are, for steel, approximately equal to 0.3 times the respective hoop tension and compression stresses` (ln the rod end 28, this axial tension stress is 0.3 times the contact pressure at the surface and 0.6 times the contact pressure on the axis of the rod end.) Consequently, an axial tension load applied to the rod 32 tends to decrease the initial axial compression stress in the coupling end 24 and to increase the initial axial tension stress in the rod and 28, the compression stress in the coupling end 24 reversing and becoming a net axial tension stress upon application of a sufficiently high axial tension load. Because of this effect, the wall thickness of the coupling end 28 can be reduced, and can be further reduced by utilizing for the coupling 22 a material having as high a strength as possible, as hereinbefore discussed. This is important beacuse it reduces the over-all diameter of the rod joint 38, which is of significance in areas, such as oil wells, where space is at a premium.

One effect of such reductions in the wall thickness Of the coupling 22 is that, when an axial tension load is applied to the rod 32, the coupling end 24, on the average, contracts more than the rod end 28. Therefore, under these conditions, the engagement pressure between the tapered surfaces 42 and 44 actually increases with an increase in the tension load, as long as the stresses in the coupling and rod ends are below the yield stresses. This is important because it increases the strength of the joint 38.

If the axial tension stress in the rod end 28 Were to exceed the tensile yield stress, with the stress in the coupling end 24 still within the elastic limit, the rod end would contract faster than the coupling end, with the result that the engagement pressure between the tapered surfaces 42 and 44 would be reduced, and with the ultimate result that the joint 38 would fail by pulling of the rod end out of the coupling end. This eifect is heightened b y the initial axial tension stre-ss in the rod end 28 induced by the shrink t between the coupling and rod ends. In the absence of any upset in the rod end 28, as the axial tension load on the rod 32 is progressively increased, the axial tension stress in the rod end 28 would rst exceed the yield strength in tension near or just within the interface between the tapered surfaces 42 and 44. The result is that the joint 38 would fail by necking down of the rod end 28, starting near the outer extremity of the coupling end 24, and progressing axially into the interior of the coupling end.

To offset the foregoing effect and thus obtain a rod joint 38 having a higher strength in tension, the rod end 28 is upset suciently to insure that the tensile yield stress in the body of the rod 32 is reached before the tensile stress in the rod end reaches the yield point adjacent or Within the outer extremity of the coupling end 24. The upset of the rod end 28 should be suicient to make the diameter of the rod end at the axial midpoint of the tapered interface between the tapered surfaces 42 1?; and 44 somewhat greater, e.g., 20% greater, than the nominal diameter of the rod.

Throughout all of the foregoing discussion of the rod joint 38, it has been assumed that the moduli of elasticity of the materials of the coupling and rod ends 24 and 28 are substantially equal. This assumption holds for coupling and rod ends of steel even if the two members are made of steels having quite-different physical properties. With the steels normally used for oil well sucker rods and couplings, the variation in modulus of elasticity will not be more than a few percent, which has only a negligiblc effect on the strength of the joint 38. However, a significant eiect can Ibe achieved by utilizing for the coupling 22 a material having a relatively low modulus of elasticity, but as high a strength as possible, and by using for the rod 32, a material having a relatively high modulus of elasticity. For example, high strength aluminum alloys might be used for the coupling 22 and steel for the rod 32. With such materials, the coupling end 24 would contract more than twice as much as the rod end 28 for a given axial tension load, thereby producing a very substantial increase in the contact pressure between the tapered surfaces 42 and 44 as the axial tension load is increased. With such a construction, joint strengths in excess of the ultimate rod body strength can readily be achieved. However, it would be necessary to utilize a relatively thick Walled coupling 22 because of the present impossibility of obtaining materials having a modulus of elasticity which is low as compared to that of steel, but having a strength which is as high as that of steel. Nevertheless, Where the wall thickness of the coupling 22 is not a factor, such a construction is entirely practical.

If the engagement pressure between the coupling 22 and the rods 32 and 34 is substantially uniform throughout the interfaces of the respective tapered surfaces, stress concentrations are produced in the rod ends at the ends of the coupling. To reduce such stress concentrations, the coupling extremities may be provided with additional tapers, either internal or external, to taper the engagement pressures toward zero at the coupling extremities. Considering first an additional internal taper, and referring to FIG. 5, the tapered surfaces 142 and 144 of the rod and coupling ends 128 and 124 are Shown as having constant and equal taper angles throughout most of the interface therebetween. However, the tapered surface 144 of the coupling end 124 is shown as provided at its extremity with an end portion 15S having, for a distance of, for example, about 25% of the engagement length, a slightly higher taper angle, eg., having an included taper angle of about 30' higher than the included taper angle of the surfaces 142 and 144. This difference in the taper angle, which is exaggerated in FIG. 5, is suicient to reduce the contact pressure to zero, or a low value, at the extremity of the coupling end 124. The resulting tapering oft of the engagement pressure toward the extremity of the coupling end 124 minimizes, or avoids entirely, any 4stress concentration in the rod end 128 at the coupling extremity. (With this means of reducing stress concentrations, the value of L in the equations hereinbefore given becomes the length of the interface between the tapered surfaces 142 and 144 up to the point where the taper of the surface 144 changes at 155.) Referring back to FIG. 1, a similar eifect may be achieved by correspondingly externally tapering the coupling end 24, as indicated at 55. As another alternative, the included taper angle of the rod end may be reduced slightly at the outer extremity of the coupling end.

The joint of the invention has previously been described as capable of being made up by application of an axial make up force without relative rotation of the inner and outer members. Shown in FIGS. 6 and 7 is an alternative rod joint construction 220 wherein the axial make up force is produced, in response to relative rotation of the rod and coupling ends 228 and 224 of a rod joint 238,

by mating wide, shallow, tapered threads 272 and 274 on the rod and coupling ends, respectively. (The necessary relative rotation may be produced by apparatus disclosed in our aforementioned patent.) The threads 272 and 274 are formed exclusively in axially-central regions of tapered surfaces 242 and 244 and have fiat root and crest surfaces which constitute the tapered surfaces 242 and 244. In other words, in the central region of the interface between the tapered surfaces 242 and 244, the interface is formed by the root and crest surfaces of the thread 272 and the mating root and crest surfaces of the thread 274. The threads 272 and 274 do not extend into the fluid-sealing regions at the ends of the tapered surfaces 242 and 244, but extend only throughout central regions thereof. The threads 272 and 274 are complementarily tapered from zero depth at one of the uid sealing regions to a maximum depth less than one-half the thread width at the other of the uid sealing regions. While the threads 272 and 274 do provide some mechanical strength, the strength of the threaded rod joint 238 of the invention is primarily due to friction between the tapered surfaces 242 and 244 as in the unthreaded species of the invention. The threads 272 and 274 function primarily to cause relative axial movement of the rod and coupling ends 228 and 224 upon relative rotation thereof while fluid under high pressure is present in the central region of the interface during making up or breaking of the joint, such uid being retained in the central region by the fluid sealing regions at the ends of the interface. Preferably, the threads 272 and 274 provide ank clearances, as shown in FIG. 7, to facilitate distribution of the injected uid throughout the central region of the interface.

With the foregoing in mind, it will be apparent that the threads 272 and 274 may be quite shallow. For example, maximum depths ranging from 0.010 inch to 0.030 inch are adequate in small-diameter rods, although they may be deeper for larger sizes. Preferably, the threads are modified square threads, actually trapezoidal threads, the anks of which include angles of the order of Typical dimensions for the rod joint construction of the invention when incorporated in an oil well sticker rod string will now be given merely for purposes of illustration. FIG. 1 of the drawing shows the rod joint construction 20 drawn to scale to represent the true proportions of the joint construction when applied to sucker rods 32 and 34 having a nominal diameter of 0.875 inch. Considering the rod joint 38 as typical, the rod 32 is upset to approximately 1.05 inches in diameter at the axial midpoint of the tapered surface 42, this upset being much less than that required in any ordinary threaded sucker rod joint wherein the entire load is taken mechanically by the threads. The included taper angle of the surfaces 42 and 44 is 4 and the material of the coupling 22 is selected to provide a yield strength about 50% above that of the material of the sucker rod 32. The outer diameter of the coupling 22 is approximately 1.56 inches, which again is a smaller value than the corresponding dimension of an ordinary threaded suckre rod joint. Assuming a surface treatment for the tapered surfaces 42 and 44 such as to give an effective coeicient of friction of 0.2, the length of pressural interengagement of the tapered surfaces required to provide the rod joint 38 with a pull-out strength equal to the yield strength of the rod 32 is approximately 1.96 inches. This results in an overall length for the coupling 22 of not more than about 5 inches, which is of the same order of magnitude as the coupling of an ordinary threaded sucker rod joint which relies on the mechanical action of the threads, and not on friction.

Although exemplary embodiments of the invention have been disclosed herein for purposes of illustration, it will be understood that various changes, modifications and substitutions may be incorporated in such embodiments without departing from the spirit of the invention.

We claim as our invention:

1. A high-strength, friction-type joint for connecting each of two elongated solid members, which are arranged in axially aligned, end-to-end relation, to a coupling member for coupling said elongated solid members together in said relation, said joint including:

(a) telescopable inner and outer members one of which is an end portion of one of said elongated solid members and the other of which is an end portion of said coupling member;

(b) an inner tapered surface within said outer member;

(c) a complementary outer tapered surface on said inner member;

(d) said tapered surfaces tapering at corresponding small angles in the direction of the common axis of said inner and outer members and being pressurally interengaged along a correspondingly tapered interface therebetween with a high engagement pressure induced by a high hoop tension stress in said outer member and an opposing high compression stress in said inner member;

(e) said high hoop tension stress and said opposing high compression stress respectively being close to but slightly less than the yield points of the materials of said outer and inner members; and

(f) the axial length of said tapered surfaces in pressural interengagement with each other being so related to the engagement pressure between said tapered surfaces and the effective coefficient of friction therebetween as to produce a frictional resistance to relative bodily displacement of said tapered surfaces sufficiently high to develop a joint strength at least nearly equal to the yield strength of said one elongated solid member.

2. A high-strength, friction-type joint for interconnecting an elongated solid member and an outer tubular member telescopable over an end portion of said solid member, including:

(a) an inner tapered surface Within said outer member;

(b) a complementary outer tapered surface on said end portion of said solid member;

(c) said tapered surfaces tapering at corresponding small angles in the direction of the common axis of said members and being pressurally interengaged along a correspondingly tapered interface therebetween with a high engagement pressure induced by a high hoop tension stress in said outer member and an opposing high compression stress in said end portion of said solid member;

(d) said high hoop tension stress and said opposing high compression stress respectively being close to but slightly less than the yield points of the materials of said outer and inner members;

(e) the axial length of said tapered surfacesy in pressural interengagement with each other being so related to the engagement pressure between said tapered surfaces and the effective coefficient of friction therebetween as to produce a frictional resistance to relatively bodily displacement of said tapered surfaces sufficiently high to develop a joint strength at least nearly equal to the yield strength of said solid member; and

(f) the engagement pressure between said tapered surfaces being not more than one-half the compressive yield stress of the material of said end portion of said solid member.

3. A high-strength, friction-type joint for interconnecting two solid rods in axially aligned, end-to-end relation, including:

(a) an outer tubular member connected to an end of one of said rods;

(b) an inner member comprising an end portion of the other of said rods;

(c) an inner tapered surface within said outer member;

(d) a complementary outer tapered surface on said inner member;

(e) said tapered surfaces tapering at corresponding small angles in the direction of the common axis of said members and being pressurally interengaged along a correspondingly tapered interface therebetween with a high engagement pressure induced by a high hoop tension stress in Said outer member and an opposing high compression stress in said inner member;

(f) said high hoop tension stress and said opposing high compression stress respectively being close to but slightly less than the yield points of the materials of said outer and inner members;

(g) the axial length of said tapered surfaces in pressural interengagement with each other being so related to the engagement pressure between said tapered surfaces and the effective coeicient of friction therebetween as to produce a frictional resistance to relative bodily displacement of said tapered surfaces sufficiently high to develop a joint strength at least nearly equal to the yield strength of said rods; and

(h) the engagement pressure between said tapered surfaces being not more than one-half the compressive yield stress of the material of said end portion of said other rod.

4. A high-strength, friction-type joint for interconnecting a solid rod and a coupling, including:

(a) an outer tubular member comprising an end portion of said coupling;

(b) an inner member comprising an end portion of said rod;

(c) an inner tapered surface within said outer member;

(d) a complementary outer tapered surface on said inner member;

(e) said tapered surfaces tapering at corresponding small angles in the direction of the common axis of said members and being pressurally interengaged along a correspondingly tapered interface therebetween with a high engagement pressure induced by a high hoop tension stress in said outer member and an opposing high compression stress in said inner member;

(f) said high hoop tension stress and said opposing high compression stress respectively being close to but slightly less than the yield points of the materials of said outer and inner members;

(g) the axial length of said tapered surfaces in pressural interengagement with each other being so related to the engagement pressure between said tapered surfaces and the effective coeicient of friction therebetween as to produce a frictional resistance to relative bodily displacement of said tapered surfaces sufficiently high to develop a joint strength at least nearly equal to the yield strength of said rod; and

(h) the axial length of the interface between said tapered surfaces being at least equal to L in the equation where SR is the axial tensile yield stress of the body of said rod,

SC is the hoop tension yield stress of said coupling,

dR is the diameter of the body of said rod,

d1 is the outer diameter of said coupling at the axial midpoint of said interface,

d2 is the inner diameter of said coupling at the axial midpoint of said interface,

f is the effective coefficient of friction betwen said tapered surfaces, and a is the included taper angle of said tapered surfaces.

5. A high-strength, friction-type rod joint according to claim 4 wherein the axial length of the interface between said tapered surfaces is at least equal to L, but not more than 3L, in said equation.

6. A high-strength, friction-type rod joint according to claim 5 wherein .SR/SC is not greater than one.

7. A high-strength, friction-type rod joint according to claim 5 wherein .SR/SC is less than one.

8. A high-strength, friction-type rod joint according to claim 4 wherein the engagement pressure between said tapered surfaces does not exceed one-half the compressive yield stress of the material of said end portion of said rod.

9. A high-strength, friction-type rod joint according to claim 4 wherein said axial length of said tapered surfaces in pressural interengagement with each other is between about 0.5 and about 6.0 times the outside diameter of the body portion of said rod.

10. A high-strength friction-type rod joint according to claim 9 wherein said effective coefficient of friction between said tapered surfaces is between about 0.1 and about 0.8.

11. A high-strength, friction-type rod joint as defined in claim 4 including mating, wide, shallow, tapered threads on said inner and outer members exclusively in the axiallycentral region of said tapered surfaces thereof for relatively axially moving such members during make-up and breaking of said joint, said inner tapered surface in said central region being formed by alternate crest and root surfaces of the threads of said outer member, said outer tapered surface in said central region being formed by alternate crest and root surfaces of the threads of said inner member, the interface in said central region being between the root and crest surfaces of the threads on either member and the root and crest surfaces of the threads on the other member, said threads functioning primarily to cause relative axial movement of said members upon relative turning thereof while fluid under high pressure is present and retained in said central region during make-up or breaking of said joint by huid-sealing regions of said tapered surfaces at opposite ends of said central region.

12. A high-strength, friction-type rod joint as defined in claim 11 in which said threads are complementarily tapered from zero depth at one of said fluid-sealing regions to a maximum depth less than one-quarter the thread pitch at the other of said fluid-sealing regions.

13. A frictional connection between two axially aligned rods having a strength at least nearly equal to the yield strength of the body portions of said rods, including a tubular coupling having ends into which adjacent solid ends of said rods are respectively telescoped in frictional engagement therewith, the rod ends having outer tapered surfaces converging axially toward each other within said coupling, the coupling ends having complementary tapered inner surfaces respectively in frictional engagement with said outer tapered surfaces of said rod ends along complementarily tapered interfaces therebetwen said outer tapered surfaces of said rod ends respectively engaging said inner tapered surfaces of said coupling ends along said tapered interfaces therebetwen with high engagement pressures induced by high hoop tension stresses in said coupling ends and opposing high compression stresses in said rod ends, said high hoop tension stresses and said opposing high compression stresses respectively being close to but slightly less than the yield points of the materials of said coupling ends and said rod ends, the axial lengths of said tapered interfaces being so related to the engagement pressures between said outer and inner tapered surfaces throughout said tapered interfaces and the effective coeicients of friction between said outer and inner tapered surfaces throughout said interfaces as to produce frictional resistances to relative bodily displacements of said 2,992,479 7/ 1961 Musser et al 29-421 outer and inner tapered surfaces at least nearly equal to 3,142,901 8/1964 Bodine 29-525 the yield strength of said body portions of said rods, said 2,899,806 8/ 1959 Fye 29-427 X rod ends being axially separated within said Coupling t0 2,926,940 3/1960 Maass 29-427X provide a space therebetween, and said coupling being 5 3,114,566 12/ 1963 Coberly et al. 285-18 provided therein with a bleed port in communication with said space between said rod ends. FOREIGN PATENTS 825,766 12/1959 Great Britain.

10 THOMAS F. CALLAGHAN, Primary Examiner References Cited UNITED STATES PATENTS 1,067,516 7/1913 Gleeson 285-13 U.S. Cl. X.R. 2,671,949 3/1954 Welton 29-148.2 287-126 patent No. 3,508,773 Dated April 2a, 1970 Inventor(s) Clarence J. Coberly and Francis Barton Brown It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 5, line 60, "nterenaged" should be nterengaged-;

Column 6, line 4, "rod'l should be --rod end;

column s, une 54, "Ss'should be "Sc-- Column ll, line 26, "and 28" should be --end 28;

Column ll, line 30, "end 28" should be --end 24;

Column 13, line 5l, "any" should be an;

Column 13, line 59, "Suckre" should be Sucker;

Column 14, line 60, "relatively" should be relative;

Column 16, line 58, "complementary" should be complementarily Under References Cited add:

3 EALED SEM-m (SEAL) Hmm E. mm, JR. Au Gomissioner of Patents .J

Edward M. Fletcher In Anesng Officer 

